A muntin is a strip of wood or metal separating and holding panes of glass in a window. Muntins can be found in doors and windows of certain styles of western architecture. The combination of muntins and glass creates a grid system dividing a single sash or casement into smaller panes, called “lights” or “lites”. Until the middle of the 19th century, large panes of glass were so expensive to manufacture that it was economically advantageous to combine multiple smaller panes of glass in a grid for the manufacture of windows and doors having large light-transmissive expanses.
So-called true divided-light residential windows typically make use of thin muntins which range from ½-inch to ⅞-inch in width. In windows constructed with wood framing, a fillet is cut into the outer edge of the muntins so as to provide a seat for a pane of glass within each opening of the grid. Putty or thin strips of wood or metal are then used to hold each pane in place. The inner sides of wooden muntins are typically milled to traditional profiles. In the U.S., the thickness of window muntins has varied historically, ranging from very slim muntins for use in 19th-century Greek revival buildings to thick muntins for use in 17th- and early-18th-century buildings.
In spite of the fact that the muntins of divided-light windows interfer with the view, many consider such windows to be more architecturally attractive than those of the single-pane variety. In addition, divided-light windows are inextricably connected with particular styles of architecture, and any attempt to build in these styles using single-pane windows is likely to be viewed not only as an exercise in poor taste, but as a cheap substitute for the genuine article. It would be unimaginable, for example, that anyone having even a modicum of architectural sensibility would attempt to build a colonial style structure using single-light windows. In addition, restoration of period-built structures requires adherence to architectural rules of that period.
The problem of maintaining architectural authenticity was compounded by the oil energy crisis of the 1970's that resulted from an oil embargo initiated by certain petroleum producing countries in 1973 in response to U.S. support for Israel during the Yom Kippur War. Single-pane windows suddenly became obsolete because their insulative R value was a mere 0.9. For structures having large expanses of single-pane windows—and this characteristic typically applied to colonial style buildings—this abysmal R-value rating guaranteed unacceptable levels of heat transfer from the interior to the exterior during winter months and in the opposite direction during summer months. Although single-pane windows are well-adapted to traditional muntins, double-pane windows are more than twice as heavy because a thick gasket, positioned around the edges of the assembly, not only maintains the separation of the double panes, but also seals the dead space between them. A true divided-light, double-pane window would therefore require the use of much heavier and considerably more expensive muntins. In comparison to the muntins of a divided-light window made of single panes, such muntins would, indeed, appear ungainly.
In an effort to avoid the high cost and unattractive appearance of true divided-light double-pane windows, window manufacturers began to supply double-pane windows having bar grills positioned between the spaced-apart double panes. Alternatively, they produced removable contoured grills which, generally, were attachable to the interior side of the double-pane glazing. Under certain, unexceptional lighting conditions, fake muntins of both types can be detected from great distances by casual observers. In fact, fake muntins have come to be regarded as a halmark of tawdry construction.
The only reasonable alternative to prohibitively-expensive and ungainly true divided-light double- and triple-pane windows, on one hand, and fake muntin grills on a large, single-light window, on the other, is a window structure accurately described as a simulated-authentic, divided-light window. The structure consists of a double- or triple-glazed sash having an authentically-detailed muntin assembly adhered to each side thereof. Muntin assemblies exposed to the elements are generally made of aluminum extrusions or other non-weathering elements. A final and essential element of the simulated-authentic window structure is the inclusion of a spacer grid between each adjacent pair of glass panes. The spacer grid not only strengthens the multi-pane sash, but also makes it appear—even on close inspection—that both muntin assemblies and the spacer grid(s) are unitary and physically separate the individual lights. Any multi-light window having any less a structure appears cheap and fake.
A major drawback associated with true, divided-light windows of both single- and multi-pane varieties is the intensive manual labor required both to clean the panes of the individual lights several times each year, as well as to paint the muntins every five to ten years. Simulated divided-light windows suffer from the same drawback if the muntin assemblies are permanently adhered to the outer surfaces of the sash. It has been estimated that the time required to clean both sides of a divided-light window can be as much as ten times that required to clean a single-light windows of the same size. The spray painting of removable muntin grids is a simple task, whereas the painting of a muntin grid without removal requires extensive masking and/or careful painting with a small brush. Thus, simulated-authentic, divided-light windows having removable outer muntin assemblies have a tremendous advantage over windows which are not temporarily convertible to single-light windows, both from the standpoint of manufacturing cost and the cost of ongoing maintenance. Nevertheless, removable muntin assemblies must be reliably secured to the sash. Muntin assemblies attached to the exterior surface of a sash must withstand exposure to the wind and the elements. If a mechanical muntin securing mechanism is employed, it must not degrade significantly over time. If one were to fall off the sash, it could not only pose a danger to a nearby object or person, but could also be damaged by impact with the ground.
A number of methods for constructing simulated divided-light windows have been devised over the years which have transpired since the Yom Kippur War. U.S. Pat. No. 4,783,938 to Douglas J. Palmer, for example, discloses a simulated divided-light window structure that is compatible with insulated double-glazed windows. The structure makes use of an internal grid structure and two external grid structures to achieve a realistic divided-light appearance. This structure does nothing to reduce the labor required to clean such windows, as both external grid structures are adhesively bonded to the glass panes. U.S. Pat. No. 5,077,950 to Donald D. Bretches, et al., on the other hand, discloses a simulated divided-light window structure for single-glazed windows, in which a pair of simulated muntin grids are attached to opposite sides of a pane through mutual magnetic attraction. There are several problems associated with this structure. The first is that the method is inapplicable to simulated muntin grids fabricated from hollow extruded aluminum members, as the method requires a magnetic or ferromagnetic strip to be adhesively bonded within groove formed on the back side of each muntin member. The method is also subject to adhesive bonding failure over time. Furthermore, the method will not work with double pane windows, as insufficient magnetic force is generated by a permanent magnet to bridge the gap between the panes.
What is needed is an improved structure for authentically simulating true divided-light windows. As with the U.S. Pat. No. 4,783,938, the structure must employ an internal grid structure in combination with a pair of external grid structures in order to achieve authentic simulation. However, the external grids structures must not only be secured by means which will not degrade over time, but they must be readily and quickly removable to facilitate window cleaning operations.